An automatic transmission mounted on a vehicle typically includes: a plurality of planetary gear sets (planetary gear mechanisms); and a plurality of hydraulic friction engaging elements such as clutches and brakes. By selectively engaging the friction engaging elements via hydraulic control, power transmission paths passing through the planetary gear sets are switched, and as a result, a plurality of forward gear stages and typically one reverse gear stage can be realized.
For example, PTL 1 discloses an automatic transmission which includes three single pinion type planetary gear sets and five friction engaging elements and realizes six forward gear stages and one reverse gear stage by engaging any two of the friction engaging elements.
On the other hand, in recent years, to improve fuel efficiency of an engine and gear shifting performance, a further increase in the number of forward gear stages has been desired. For example, an automatic transmission is considered, which includes three planetary gear sets and six friction engaging elements and realizes eight forward gear stages by combinations each realized by engaging any two of the friction engaging elements.
According to this configuration, four friction engaging elements which are not engaged exist at each gear stage. Therefore, there is a possibility that due to sliding resistance between friction plates of the friction engaging elements, viscous resistance of lubricating oil between the friction plates, and the like, a driving loss of the entire transmission will become large, and an improvement effect of the fuel efficiency by the increase in the number of gear stages will deteriorate.
PTL 2 discloses an automatic transmission which includes two single pinion type planetary gear sets, two double pinion type planetary gear sets, and five friction engaging elements and realizes eight forward gear stages by selectively engaging any three of the friction engaging elements.
According to this configuration, the number of friction engaging elements which are not engaged at each gear stage is two. Therefore, a driving loss such as that above is reduced. In addition, since the double pinion type planetary gear set with a higher degree of freedom in setting a change gear ratio compared to the single pinion type planetary gear set is included, a merit of being able to appropriately set the distribution of gear steps (a reduction ratio of a lower gear stage/a reduction ratio of an upper gear stage) between the adjacent gear stages can be expected.